In charged particle systems, comprising both electron microscopes and focused ion beam systems, a column is typically used to focus a charged particle beam onto the surface of a target to be imaged and (optionally) processed using the beam. To form an image of the target, it is necessary to deflect the beam across the target surface, usually in a raster pattern. Due to the impact of the charged particle beam with the target, secondary particles are emitted and may be collected to form an imaging signal. As an example, an electron beam will stimulate the emission of secondary electrons from the target. A focused ion beam will stimulate the emission of both secondary electrons and secondary ions (usually positively-charged). A secondary particle detector is employed in these systems to generate the necessary imaging signal—these detectors may be characterized by their collection efficiency, i.e., the fraction of emitted secondary particles which are actually collected by the detector. To enhance this collection efficiency, a “collection” grid is often positioned between the target and the detector. A voltage applied to this grid creates an electric field between the target and grid to attract secondary particles, which then pass through the grid (a sparse mesh or other nearly transparent structure) and then to the detector.
Unfortunately, it is found that over time, secondary detectors may exhibit a loss in efficiency due to either damage to the detector and/or a build-up of contamination on the detector surface. This damage results from the energetic bombardment of the detector by incoming secondary particles, which can disrupt the detector material. Contamination covers the detector with a thin film such as polymerized hydrocarbons arising from the interaction of the charged particle beam with trace gases in the vacuum system—often these gases arise from the interaction of the charged particle beam with the target, and thus are difficult to avoid even in systems with very low base pressures. Typically this damage is non-uniform over the detector surface. This non-uniformity arises because the emission pattern of secondary particles from the target is concentrated in a direction upwards (following a cosine law) from the target. If the detector is an annulus surrounding the primary charged particle beam, then the majority of the collected secondary particles will strike the detector near the center. Thus, the accumulated damage and/or contamination on the detector will also be concentrated near the center. Detector lifetime is determined by the most damaged or contaminated area (even if the majority of the detector area is still functional), so when the center of the detector becomes unusable due to damage and/or contamination, the entire detector must be replaced, reconditioned, or cleaned. Thus, it would be advantageous in charged particle systems to improve the detector lifetime by making the damage and/or contamination rate more uniform over the area of a charged particle detector to improve the overall detector lifetime.